This is not just for testing SQL (which it could be handy for), but for sharing. Suppose you have a scenario where you have a few tables, and you run some SQL and it's not giving the result you expect. You can create a SQL fiddle, post it on your favorite Q/A site, and other SQL experts can quickly tweak your SQL to work right, and share the result back with you.

I'd use this tool.

Testing out snippets of code without altering a structure of the database you have installed locally/on a server.

Doesn't require a startup of the database - the results can be discarded and you don't have to worry about cleaning it up :)

Generally, just a fast way to test things - I would see it coming in handy during some presentations maybe?

the same point as JSFiddle.net I presume

http://css-tricks.com/seriously-just-make-a-jsfiddle/